Many semiconductor fabrication methods employ plasma to perform etching processes where material on a wafer is removed in specific areas to subsequently form the components/features of the devices (e.g., transistors, capacitors, conductive lines, vias, and the like) on the wafer. The fabrication methods use a mask pattern that is formed over areas of the wafer that are to be protected from the etching process.
During etching of deep features requiring long plasma exposure times, the mask pattern may be completely removed from the wafer surface and thereby leave the surface unprotected. Therefore, etching of deep features on a wafer can be limited by the etch selectivity between the material of the mask pattern and the material to be etched, where higher the selectivity, the deeper the feature may be etched. Furthermore, etching of deep features generally requires straight feature sidewalls and high etch selectivity to material at the bottom of the features.
Silicon nitride (SiN) films are widely used in microfabrication processes as a dielectric and mask material. Semiconductor processing often involves etching features in a relatively thick layer of SiN film on a Si wafer substrate or on a relatively thin layer of silicon dioxide (SiO2) supported upon a Si wafer substrate, where high selectivity of SiN etching over both Si and SiO2 is strongly desired to reduce or prevent damages in an underlying SiO2 film or Si substrate.
There is a need for new methods for increasing the selectivity during etching of deep SiN features with straight sidewalls, such that a sufficient portion of the mask pattern remains to cover areas of the wafer to be protected until the etch process is complete and such that the underlying substrate materials are not etched or damaged.